Viscosifying water-soluble polymers are used in various fields, such as paints, glues and adhesives, the building industry, the textile industry and the paper industry.
The aqueous and/or pigmented compositions with which a person skilled in the art is often concerned, for example aqueous paints, are composed of a liquid phase, which can be water or a mixture of water with a water-miscible organic solvent, of a polymer dispersed in the liquid phase, commonly known as "binder", of fillers and/or pigments, of an agent for dispersing fillers and/or pigments, which can be a water-soluble polymer, and of various adjuvants, such as coalescence agents, biocides, foam-suppressants or others, and finally of one or more viscosifying (or thickening) agents which are natural or synthetic polymers.
Polymers generally increase the viscosity of the solutions in which they are dissolved. The water-soluble polymers generally used as viscosifying agents for aqueous solutions have varied structures; mention may be made of polyacrylamides, optionally partially hydrolysed, of poly(sodium (meth)acrylate)s and their copolymers, of cellulose derivatives, of poly(ethylene oxide)s or alternatively of polysaccharides.
However, the use of conventional water-soluble polymers remains limited. This is because, in order to have good viscosifying properties, the water-soluble polymer must have a high molecular mass. However, the majority of the industrial applications cited above require subjecting the aqueous solution containing the viscosifying polymer to a high shear gradient. This often causes the polymer to degrade, leading to a reduction in its molecular mass and a decrease in its viscosifying power.
Low-mass polymers are certainly less sensitive to mechanical stresses but they have to be used at high concentrations, which is generally incompatible with industrial applications.
Natural polymers, such as cellulose derivatives, are, for their part, sensitive to microbial attacks and require the addition of antimicrobial agents.
In order to improve the performance of water-soluble polymers, associative polymers, which, according to the definition given of them in the Encyclopedia of Polymer Science and Engineering, 2nd Edition, 17, 772-779, are water-soluble polymers containing non-polar groups which gather together in aggregates in polar media, have been developed. They are composed of a skeleton containing predominantly units with a hydrophilic nature and, to a minor extent, hydrophobic sequences. When such structures are dissolved in aqueous solutions, their hydrophobic centres gather together in order to limit the water/hydrophobic sequence interactions. The formation of such physical crosslinking nodes can result in the creation of a true network. The physical gel thus formed considerably increases the viscosity of the water. In the case of an aqueous composition containing fillers and/or pigments, the associative polymers also act by the creation of various bonds between themselves and certain constituents of the compositions. They then contribute, to the compositions containing fillers and/or pigments to which they are added, a theological behaviour which is less pseudoplastic and which thus facilitates the applications.
Associative polyurethane thickeners and associative acrylic thickeners are particularly well known among associative polymers.
Associative polyurethane agents are copolymers with an essentially triblock structure, that is to say molecules composed of three distinct parts, the polymerized hydrophilic central part and two ends, identical or otherwise, composed of hydrophobic groups, such as, for example, alkyl, aryl or alkylaryl groups. Such agents are described in numerous patents, such as, for example, U.S. Pat. Nos. 3,770,684, 4,079,028 and U.S. Pat. No. 4,155,892.
Associative acrylic agents have a different structure based on a main chain with a hydrophilic nature, along which pendant hydrophobic units are randomly distributed. They are most often obtained by emulsion polymerization in water.
Associative polyacrylics only develop their thickening or viscosifying power in alkaline medium. Moreover, on account of their condition of synthesis, they do not exhibit rheological properties with a threshold effect because their molecular mass is not high enough. Inverse emulsion polymerization is a method of choice well known to a person skilled in the art for obtaining acrylic polymers of high molecular mass since it combines a high reaction rate with the achievement of high molecular masses of greater than 1 million. Inverse emulsion polymerization is today widely employed industrially.
It has also been used, but much less often, for the synthesis of associative acrylic (co)polymers. Mention may be made, as example, of U.S. Pat. No. 4,921,903 which teaches the art of synthesizing associative terpolymers by inverse emulsion polymerization but in two stages. In practice, the hydrophobic group is incorporated by transamidation of a hydrophilic copolymer containing amide monomer units.
U.S. Pat. No. 4,918,123 teaches the way of introducing octylacrylamide as hydrophobic monomer in a water/oil emulsion containing units derived from a non-ionic monomer, such as acrylamide, and from a cationic monomer, such as 3-methacrylamidopropyltrimethyl-ammonium chloride. This incorporation is carried out using a third, alcohol, solvent and by using a polymerization inhibitor which is soluble in the oil phase, in order to prevent any polymerization of the hydrophobic monomer in the oil phase. The use of an alcohol is very often undesirable in the final application.
As may be observed on reading the prior art, it is not easy to produce an associative acrylic polymer of high molecular mass by the inverse emulsion polymerization technique. The processes described are of little industrial applicability, require the use of a cosolvent additive, the presence of which is sometimes troublesome to the final user, or require the preparation of specific surfactants intended solely for this use (EP 172,015 and EP 172,724), which consequently increases the cost of this type of product. Moreover, on account of the difficulty in optimizing the final stability and its aptitude for inversion, it is a very complex business to modify the polymerization parameters in order to adapt them to novel requirements as regards final properties of the polymer.
All this is in addition to the conventional disadvantages of inverse emulsions, that is to say the lack of stability over time and the broad particle size distribution.